Merge pull request #4 from TobiasLohner/master

Update xcsoar source.
Update to latest XCSoar 6.8 dev branch HEAD

This adds some bugfixes in the replay code and QNH processing.

The travis before_install is needed to initialize the boost submodules
inside xcsoar's repository (see http://docs.travis-ci.com/user/build-configuration/#Git-Submodules)
Add interface to xcsoar's airspace warning code.